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Electrodeionization (EDI) unit
Electrodeionization
I. Introduction Electrodeionization (EDI) is a technology for producing ultrapure water that integrates ion exchange, ion exchange membrane, and ion electro-migration technologies. It skillfully combines electrodialysis and ion exchange, utilizing high voltage from electrodes at both ends to mobilize charged ions in water, accelerating their removal in conjunction with ion exchange resins and selective ion exchange membranes, thereby achieving water purification. Thus, the EDI system here is a system for producing ultrapure water. During the EDI desalination process, ions are removed under the influence of an electric field through ion exchange membranes. Simultaneously, water molecules generate hydrogen ions (H⁺) and hydroxide ions (OH⁻) under the electric field, which continuously regenerate the ion exchange resins to maintain their optimal state. The historical progression of EDI ultrapure water equipment can be divided into three stages: First Stage: Pretreatment filter → cation bed → anion bed → mixed bed. Second Stage: Pretreatment filter → reverse osmosis (RO) → mixed bed. Current Stage: Pretreatment filter → RO → EDI (acid-base free). For decades, the mixed bed ion exchange technology has been the standard process for ultrapure water production. However, due to its periodic regeneration requirements, which consume significant amounts of chemicals (acids and bases) and industrial pure water, and generate environmental issues, there is a need to develop acid-base-free ultrapure water systems. As traditional ion exchange technologies increasingly fail to meet modern industrial and environmental protection demands, EDI technology, which combines membranes, resins, and electrochemical principles, has revolutionized water treatment. EDI uses electrical energy for resin regeneration instead of acids and bases, aligning better with current global environmental protection requirements. II. Working Principle The Electrodeionization (EDI) system is a scientific water treatment technology that primarily relies on a direct current electric field to induce directional migration of dielectric ions in water between spacers, utilizing the selective permeability of exchange membranes to purify water. Between a pair of electrodes in an electrodialyzer, multiple groups of anion membranes, cation membranes, and spacers (A and B) are alternately arranged, forming concentrate and diluate chambers (i.e., cations can pass through cation membranes, and anions can pass through anion membranes). In the diluate chamber, cations migrate towards the negative electrode and pass through the cation membrane, being retained by the anion membrane in the concentrate chamber; anions migrate towards the positive electrode and pass through the anion membrane, being retained by the cation membrane in the concentrate chamber. Consequently, the number of ions in the diluate chamber water gradually decreases, forming fresh water, while in the concentrate chamber, the continuous influx of cations and anions increases the dielectric ion concentration, forming concentrate water, thereby achieving the objectives of desalination, purification, concentration, or refinement. III. System Characteristics EDI systems have seen significant development in industries such as pharmaceuticals, semiconductors, power, and surface cleaning, and are also widely used in wastewater treatment, beverages, and microbiology. EDI equipment is applied after the RO system to produce stable ultrapure water, replacing the traditional mixed bed ion exchange technology (MB-DI). Compared with mixed ion exchange technology, EDI technology offers the following advantages: 1. Stable water quality. 2. Easy to achieve fully automated control. 3. No downtime due to regeneration. 4. No need for chemical regeneration. 5. Low operating costs. 6. Small plant footprint. 7. No sewage discharge. EDI Working Principle: The EDI module sandwiches ion exchange resins between anion and cation exchange membranes to form EDI cells. A certain number of EDI cells are separated by grids within the module, forming concentrate and diluate chambers. Anode and cathode electrodes are placed at both ends of the cell stack. Under the driving force of direct current, cations and anions in the diluate chamber water flow pass through the cation and anion exchange membranes, respectively, into the concentrate chamber and are removed from the diluate chamber. The water passing through the concentrate chamber carries the ions out of the system, becoming concentrate water. EDI equipment typically uses secondary RO pure water as EDI feedwater. RO pure water generally has a resistivity of 40 - 2 μS/cm (25°C), while EDI pure water can achieve a resistivity as high as 18 MΩ·cm (25°C). However, depending on the deionized water application and system configuration, EDI ultrapure water is suitable for producing water with a resistivity requirement of 1 - 18.2 MΩ·cm (25°C). IV. System Operation 1. Factors Affecting Operation (1) Effect of EDI Feedwater Conductivity Under the same operating current, as the conductivity of the raw water increases, the removal rate of weak electrolytes by EDI decreases, and the conductivity of the effluent also increases. If the raw water conductivity is low, the ion content is also low, and the low concentration of ions results in a larger electromotive force gradient formed on the resin and membrane surfaces in the diluate chamber, enhancing water dissociation, increasing the limiting current, and generating a greater quantity of H⁺ and OH⁻ ions, leading to good regeneration of the anion and cation exchange resins filled in the diluate chamber. (2) Effect of Operating Voltage-Current As the operating current increases, the quality of the produced water continuously improves. However, if the current is increased beyond its peak point, the excessive H⁺ and OH⁻ ions generated from water electrolysis, in addition to being used for resin regeneration, will have a large surplus that acts as carrier ions for conduction. Simultaneously, the accumulation and blockage of these carrier ions during movement, and even reverse diffusion, will result in a decline in the quality of the produced water. (3) Effect of Turbidity and Silt Density Index (SDI) The EDI module's water production channels are filled with ion exchange resins. Excessive turbidity and SDI can cause channel blockages, leading to an increase in system pressure differential and a decrease in water production. (4) Effect of Hardness If the residual hardness in the EDI feedwater is too high, it can cause scaling on the membrane surfaces in the concentrate water channels, reducing the concentrate water flow rate and the resistivity of the produced water, affecting the water quality. In severe cases, it can block the concentrate and electrode water channels of the module, leading to internal heating and damage to the module. (5) Effect of Total Organic Carbon (TOC) If the organic content in the feedwater is too high, it can cause organic pollution of the resins and selective membranes, leading to an increase in the system's operating voltage and a decline in the quality of the produced water. It can also easily form organic colloids in the concentrate water channels, causing blockages. (6) Effect of CO₂ in Feedwater The HCO₃⁻ generated from CO₂ in the feedwater is a weak electrolyte that can easily penetrate the ion exchange resin layer, causing a decline in the quality of the produced water. (7) Effect of Total Exchangeable Anions (TEA) High TEA will reduce the resistivity of the EDI produced water or require an increase in the EDI operating current. Excessively high operating currents can lead to an increase in system current and a higher concentration of residual chlorine in the electrode water, which is detrimental to the lifespan of the electrode membranes. Additionally, factors such as feedwater temperature, pH, SiO₂, and oxides also affect the operation of the EDI system. 2. Feedwater Quality Control (1) Control of Feedwater Conductivity Strictly control the conductivity during the pretreatment process to ensure that the EDI feedwater conductivity is less than 40 μS/cm, which can guarantee the qualification of the effluent conductivity and the removal of weak electrolytes. (2) Control of Operating Voltage-Current Select an appropriate operating voltage-current for the system. Since there is a limiting voltage-current point on the voltage-current curve of EDI water purification equipment, which is related to factors such as feedwater quality, membrane and resin performance, and membrane stack structure, the selected operating voltage-current point for the EDI water purification equipment must be greater than the limiting voltage-current point to ensure that a sufficient amount of H⁺ and OH⁻ ions are generated from water electrolysis to regenerate a certain amount of ion exchange resins. (3) Control of Feedwater CO₂ Adjust the pH by adding alkali before the RO system to maximize CO₂ removal, or use a degasifier or degasifying membrane to remove CO₂. (4) Control of Feedwater Hardness Combine CO₂ removal with softening and alkali addition for the RO feedwater. When the feedwater salt content is high, add an additional RO or nanofiltration stage in conjunction with desalination. (5) Control of TOC Combine with other index requirements and add an additional RO stage to meet the requirements. (6) Control of Turbidity and SDI Turbidity and SDI are among the main indicators for RO system feedwater control. Qualified RO effluent generally meets the EDI feedwater requirements. (7) Control of Fe Control the Fe content in the EDI feedwater to below 0.01 mg/L during operation. If the resins have already been "poisoned," acid solution can be used for resuscitation treatment, which is relatively effective. |
